The invention relates to a circuit arrangement for bias adjustment of bus levels or a system for the transmission of binary data via at least one lead whereto a plurality of stations with transmitters and receivers with preceding low-pass filters are connected, which lead carries a recessive level and can be switched to a dominant level by a transmitter.
Possible fields of application of such systems are the networking of control devices or finished apparatus, notably in automotive and industrial electronics. Very strong interference occurs in such environments, for example due to the switching on of large loads, so that special requirements are imposed as regards the transmission reliability.
Until now transmission system are known in which the recessive level is adjusted by means of a resistor and the dominant level is fed by means of a driver. The impedance of the driver is lower than the resistance, so that interference signals which could unduly imitate a recessive signal are less likely to occur in the case of the dominant level than interference signals which could imitate a dominant signal in the case of recessive levels.
In order to avoid transmission errors due to ground offset and electromagnetic radiation, as large as possible level differences are defined. The level difference, however, should not be too large, as otherwise radiation takes place due to large level differences.
Low impedance feeding of the recessive level imposes an even lower impedance feeding of the dominant level, resulting in larger currents which cause more radiation. When the recessive level is fed with a resistor and radiation takes place, the current increases further after having reached the receiver threshold. This leads to unnecessarily high radiation. The current is unnecessary because the resistance to radiation cannot be further improved by the current increased beyond the receiver threshold.
Therefore, it is an object of the invention to provide an arrangement which improves the resistance to radiation also for the recessive level.
This object is achieved in that the stations are provided with a termination arrangement which provides low-impedance termination of the lead in the case of a level beyond the dominant level.
The recessive level is permanently adjusted on the lead by all participating stations. The working point of the recessive driver lies at, for example 0 V. When the radiation levels increase beyond the working point of the driver, the current may increase again without affecting the current in the working point which is relevant to the radiation. Thus, a lower resistance is also achieved for high radiation levels in the first quadrant. Because the impedance of the driver is lower for positive voltages than for negative voltages, in the case of radiation during a recessive level the direct voltage level will preferably move even more clearly in the direction of recessive levels, so that the logic evaluation remains recessive in any case. The mean voltage value moves towards recessive, because the voltages coupled in by a disturbance are attenuated more in the range of the positive voltage than in the range of the negative voltages. The dominant level is not affected thereby, because the impedance of the driver is usually very low.
Preferably, the circuit arrangement includes a current source termination arrangement which optimizes the ratio of the currents on the bus, and the inherent radiation, to the speed at which the recessive level is adjusted.
A current source arrangement in the termination arrangement creates a constant current for the recessive levels in the case of radiation in the range from a voltage below the receiver threshold of the receiver to a voltage beyond the working point of the driver for the dominant level. It is thus achieved that the current does not increase linearly, and hence increases the radiation, during the recessive level and radiation. In the case of small amounts of radiation, the current on the lead increases linearly up to a voltage which lies below the receiver threshold. The rise of the current is then steeper than the rise of the current in the case of termination of the lead by means of a resistor.
In systems in which data is transmitted by means of two levels, i.e. a recessive and a dominant level, the speed at which the recessive level can be adjusted essentially defines the maximum transmission rate that can be achieved. The evaluation of the logic level at the receiving side is performed by means of the receiver and a preceding low-pass filter with a predetermined receiver threshold.
EP 0 288 740 describes a system in which the time for switching over between two binary level states is reduced by means of a trigger circuit which is connected to a bus lead. Therein, the bus lead is subdivided into lead segments, all lead segments being combined via logic gates in such a manner that switching over is accelerated.
In order to switch over the bus capacitance sufficiently quickly, thus achieving adequate speed for the desired data transmission rate notably during the adjustment of the recessive level, the difference between the levels must satisfy given criteria. According to the method in which the recessive level is adjusted by means of a resistor, the bus capacitance is first switched over with a large current and subsequently with a continuously decreasing current, i.e. in conformity with the linear current/voltage ratio at the resistor. In the current-limited termination arrangement according to the invention there is initially a constant current from the direction of the dominant level up to the receiver threshold. In the lower voltage range, near the zero point, the current-limited termination arrangement has a resistance which is lower than that in the case of termination by means of a linear resistor. The initially slower switching over of the bus capacitance, due to the current limitation, is thus compensated by the comparatively faster switching over near the end of the switching operation. Overall, equally fast or even faster switching over is thus achieved in comparison with that achieved by means of a linear resistor. The total current in the static dominant state is then significantly smaller so that the radiation is also less. At the same time, however, the resistance to radiation in the recessive state is better than in the case of a resistor, because overall the impedance of the termination of the recessive level overall is lower.
For the whole range of radiation to be expected, the impedance in the first quadrant in the current/voltage characteristic of the current-limited termination device is lower than that in the third quadrant.
The first quadrant of lower impedance, moreover, also has a favorable effect on the ground offset between the individual stations in said systems which occasionally occurs due to operating conditions or aging phenomena. In combination with the static receiving threshold such ground offset could lead to a less favorable signal-to-noise ratio. In systems in which ground offset occurs and the recessive level is adjusted by means of linear resistors, a recessive level is formed between the different ground potentials, that is to say with a weight of the linear resistors distributed across the stations. The receiving thresholds of the individual stations are always related to local ground and hence differ as much as the ground potentials. The signal-to-noise ratio thus becomes different for the stations and the resistance to radiation is reduced, so that incorrect detection of dominant levels could occur in receivers with such a ground offset.
When the recessive level is adjusted by means of a termination arrangement according to the invention, where the third quadrant is of higher impedance, or outputs less current, than the first quadrant, the recessive level is in principle adjusted nearer to the lower ground potential and overall a better signal-to-noise ratio is obtained for the stations.
The resistor is replaced by a termination arrangement and the current limitation at the individual stations is adjusted in such a manner that the sum of all currents enables the desired speed of adjustment of the recessive level while at the same time the current capability of the driver is not exceeded. The rises of the individual segments of the current/voltage characteristic can be programmed by parameter modification in such a manner that the recessive level is optimally sustained in the case of particularly frequent radiations. The possibility for integration of the current source represents a further advantage.
The current/voltage characteristic represents the relevant variation when the transmission lead is subjected to radiations which vary the current and the voltage on the line in the case of a normally recessively imposed level. The current/voltage characteristic of the termination arrangement for the recessive level is, for example a characteristic with three break points.
All steps and advantages can be used for differential systems if they are applied to two bus leads which are driven in a complementary fashion. The behavior of the driver for the second bus lead is symmetrical with respect to that for the first bus lead. The driver for the second bus lead thus has a higher impedance in the first quadrant than in the third quadrant. In the differentially operating systems a comparatively higher common mode range is generally tolerated so that a high resistance to radiation is possible.